The brightness of a microscopic image normally changes when switching objectives on an objective turret. This brightness change is caused by different transmission characteristics of the objectives. The brightness change is especially intense if there is a change between different microscopic methods such as between bright field and dark field illumination.
In view of the above, the observer at the ocular can be subjected to glare during the switchover operation. Account must be taken of significantly different illuminating times for a photographic recording. When the microscopic image is to be recorded with a television camera, the danger is present that the camera will be overdriven and that irreversible damage will occur to the video tube.
To solve this problem, published German patent application DE 32 21 804 discloses an arrangement wherein the light intensity of the microscopic intermediate image is measured with a detector and the lamp voltage of the microscope illumination is correspondingly adjusted. However, this arrangement has the disadvantage that it changes the spectral composition of the light together with the intensity thereof. In this way, the color temperature of the intermediate image changes. This is especially disturbing for microphotographic recordings.
So-called CTV-objectives are also known which are matched to each other with respect to their transmission characteristics with the aid of an absorbing coating on a lens. In this way, a constant image brightness is made possible when switching over the objectives without a change of the color sensation. However, these objectives have been shown to be disadvantageous for the microscopy of objects with very weak light. The light which is already weak is further weakened by the absorbing layer. The possibilities of using these objectives are therefore very limited.
U.S. Pat. No. 4,661,692 discloses a very complex arrangement for equalizing image brightness. This arrangement includes a memory wherein the optical data of the objectives, condensers and filters are stored. The data of the objective which is to be switched into the imaging beam path is read out by a computer from a memory. An optimal condenser is selected in correspondence to these data and brought into the beam path. The computation of the light intensity required for a constant image brightness is computed from the combination of the data of the objective and from the data of the condenser. This light intensity is then adjusted in the best possible manner in that a combination of up to four absorption filters are placed in the beam path of the microscope illumination with these filters being binarily graduated with respect to each other.
Notwithstanding the great technical complexity required for this arrangement, the image brightness equalization takes place with an error which can amount to 50% because of the graduation of the absorption filters.
German utility model registration DE-GM 85 30 473 discloses a microscope having a reflector slider carrying different illuminating reflectors. Another slider which carries neutral density filters is arranged in the beam path parallel to the reflector slider and can be coupled to the reflector slider. In a simple manner, this arrangement makes possible an image brightness equalization which can also be switched off when exchanging illuminating reflectors; that is, when there is a switchover between different contrast methods such as between bright field and dark field. However, an image brightness equalization in the context of an objective switchover on a turret does not take place with this arrangement.